Oxidation of secondary alcohols

ABSTRACT

An apparatus for the molecular oxygen oxidation of a secondary alcohol such as isopropanol to hydrogen peroxide is provided.

This is a division, of application Ser. No. 08/268,891, filed Jun. 30,1994 now U.S. Pat. No. 5,552,131.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the oxidation of a secondary alcoholsuch as isopropanol to form hydrogen peroxide and to an improved methodand apparatus for carrying out this oxidation.

2. Description of the Prior Art

Methods are known for the production of hydrogen peroxide by themolecular oxygen oxidation of a secondary alcohol at conditions ofelevated temperature and pressure. See, for example, U.S. Pat. No.2,871,101 to Rust, et al., U.S. Pat. No. 2,871,102 to Rust, et al., U.S.Pat. No. 2,871,103 to Skinner, et al., U.S. Pat. 2,871,104 to Rust, andthe like.

There are problems associated with the secondary alcohol oxidation. Thereaction is exothermic and involves mixing and handling potentiallyflammable materials. Selectivity to the desired hydrogen peroxideproduct has been lower than desired. A consideration in the oxidation oflower secondary alcohols to form hydrogen peroxide is that the partialpressure of oxygen in the vapor above the liquid reaction mixture mustbe relatively high in order to achieve good reaction selectivities. Thisrequirement precludes simple purging or venting or effluent gases afterseparation of condensibles and also poses considerable flammabilityproblems in handling effluent gases from the reactor. The presentinvention provides a process and apparatus for carrying out the reactionat high selectivity while minimizing problems associated with thereaction system.

BRIEF DESCRIPTION OF THE INVENTION

In one aspect, the invention relates to an improved method for providingoxygen to the reaction system. Rather than sparging oxygen into thereactor as has been suggested by prior workers, in accordance with theinvention oxygen is added to a circulating reaction mixture stream whichis circulated at high velocity and the resulting liquid reaction mixtureand dispersed oxygen is passed to the reactor thereby providing a highrate of mixing and dispersion.

In another aspect, an improved process and a unique reactor design isprovided by which the secondary alcohol oxidation is carried out in aplurality of distinct reaction zones.

In yet another aspect, a ballast gas is incorporated in reactoroff-gases to facilitate treatment.

BRIEF DESCRIPTION OF THE DRAWINGS

Accompanying FIG. 1 illustrates schematically practice of the inventionincluding the oxygen introduction procedure. FIGS. 2a and 2b illustratethe plural zone reaction system of the invention.

DETAILED DESCRIPTION

The general conditions for carrying out the secondary alcohol oxidationare those described, for example, in U.S. Pat. No. 2,871,104, thedisclosure of which is hereby incorporated by reference. This referencedescribes reactants and reaction conditions used in the process of thepresent invention.

Attached FIG. 1 illustrates the oxygen addition procedure of the presentinvention. Referring to FIG. 1, the liquid phase oxidation of thesecondary alcohol is carried out in reaction vessel 1. In the oxidation,it is advantageous to employ a reactor having a low surface area tovolume ratio in order to minimize reactor surface caused hydrogenperoxide decomposition and for this reason the spherical reactorconfiguration shown in FIG. 1 is preferred. The cost of a sphericalreactor is also less than one half the cost of a cylindrical reactor.Cylindrical and other shaped reactors can be employed but are lesspreferable.

As shown in FIG. 1 and described more fully hereinbelow, sphericalreactor 1 is provided with cylindrical baffle 2 which divides reactor 1into a central cylindrical zone 3 and an annular zone 4. Holes areprovided in the lower section of baffle 2 effective to permitcirculation of reaction liquid from zone 4 to zone 3.

Feed secondary alcohol such as isopropanol is introduced via lines 5into annular zone 4. The temperature in zone 4 is effectively controlledby regulating the temperature of the feed alcohol. Oxygen for theoxidation is supplied to zone 4 as part of the recycle gas streamintroduced to zone 4 via lines 6. Conditions are controlled such thatabout 10-30% of the total alcohol oxidation to hydrogen peroxide takesplace in zone 4. It is generally preferred to carry out the oxidationsuch that the per pass alcohol conversion is less than 50%, preferably10-40% in the overall oxidizer.

Selectivity to the desired hydrogen peroxide product is favored by lowconcentrations of hydrogen peroxide in the reaction mixture. It isadvantageous to maintain the hydrogen peroxide concentration in thereaction mixture in annular zone 4 at about 1-10 wt %, preferably 2-8 wt%, and the concentration of hydrogen peroxide in the reaction mixture inzone 3 at about 10-40 wt %, preferably 15-30 wt %.

The liquid reaction mixture from zone 4 flows through the holes in thelower part of baffle 2 into central cylindrical zone 3 where the bulk ofthe alcohol oxidation eg. 70-90%, takes place. Oxygen is introduced intozone 3 via line 7 along with recycle reaction liquid as later described.Temperature is controlled and the reaction exothermic heat removed byvaporizing part of the reaction mixture, removing the vapor via line 8,and condensing the condensible portion in condenser 9. Vapors from bothzones 3 and 4 exit reactor 1 via outlet 8. The cooled mixture passesfrom condenser 9 via line 10 to separator 11 and condensed liquid passesfrom separator 11 via line 12 back to zone 3 of the reactor. Uncondensedvapors are removed from separator 11 via line 13, a portion is purgedvia line 14 and remainder is passed via line 6 to zone 4 of the reactor.

As a special feature of the invention, the molecular oxygen oxidant isintroduced in a manner which substantially lessens flammability problemsnormally encountered in such systems. Specifically, liquid reactionmixture is withdrawn at an extremely high rate from zone 3 via lines 15.A net product stream comprising about 5 to 25% of the reaction mixturewithdrawn via lines 15 is removed via line 16 with the remaindercirculating via pump 17 and line 7 back to zone 3 of the reactor. Thusfor each volume of product withdrawn, 3-19 volumes of reaction liquidare recirculated to zone 3.

The total oxygen feed required for both zones 3 and 4 is introduced intothe circulating reaction mixture via line 18 whereby due to the rapidliquid circulation rate in line 7, the introduced oxygen is almostimmediately dispersed in the liquid without the formation of possiblyflammable vapor pockets. The high rate of circulation via line 7 alsoensures good mixing in zone 3 thus promoting the speed and uniformity ofthe reaction therein.

In certain technologies, exothermic heat of reaction is removed byindirect heat exchange with circulating reaction liquid. In the presentsystem, this method of heat removal with its disadvantageous effect ofhydrogen peroxide decomposition by contact with metal heat transfersurfaces can be avoided.

A further feature of the invention is the provision of a ballast gassuch as methane which has a high heat capacity and which issubstantially inert under the reaction conditions to suppress theflammability characteristics of the vapor removed from separator 3. Theballast gas is introduced via line 19 and admixed with the reactionvapor mixture in line 8. Although methane is preferred, other ballastgases which can be used include ethane and propane.

In the oxidation of secondary alcohol to hydrogen peroxide, in order toachieve good reaction rate and selectivity it is necessary to operateunder conditions such that there is a substantial partial pressure ofoxygen in the exit gas. The presence of a ballast gas such as methane inthe exit gas during subsequent treatment steps and recycle is useful inavoiding possible flammability problems. Generally, the ballast gas isadded in amount sufficient to provide a ballast gas concentration in therecycle gas of about 5-40 volume %.

The oxygen introduction system and the use of ballast gas have beendescribed in conjunction with the novel plural zone reactor depicted inFIG. 1 but is will be understood that these features are usefulgenerally in secondary alcohol oxidation to hydrogen peroxide and can beused with other known reactor configurations.

A special feature of the present invention is the provision of a novelreactor and process as illustrated in FIGS. 2a and 2b. FIG. 2a is anelevation view of the reactor while FIG. 2b is a sectional view A--A ofthe reactor. Referring to FIGS. 2a and 2b, reactor 101 is a sphericalreactor having cylindrical baffle 102 mounted therein so as to provide acentral cylindrical reaction zone 103 and an annular reaction zone 104separated by the baffle. The bottom of the baffle 102 is secured to theinner wall of reactor 101. A common vapor space is above zones 103 and104.

Baffle 102 has holes near its lower end adapted for the passage of thereaction mixture therethrough from annular zone 104 to centralcylindrical zone 103. Reactor 101 is adapted for the continuousoxidation of a secondary alcohol to form hydrogen peroxide. Secondaryalcohol is fed to zone 104 of reactor 101 via inlets 105 and distributedvia distributor 106. The oxygen containing recycle gas stream is fed tozone 104 via inlet 107 and distributed via distributor 108. Reactiontemperature in zone 104 is regulated by adjustment of the temperature ofthe feed secondary alcohol. Conditions in zone 104 are controlled toprovide about 10-30% of the total conversion of the secondary alcohol.

During operation, reaction mixture from zone 104 continuously flowsthrough the holes in baffle 102 into zone 103. Liquid reaction mixtureis removed from zone 103 via outlets 109 with a portion recovered asproduct and the predominance being recirculated after oxygen injectionas above described, the mixture of recirculated liquid and feed oxygenentering zone 103 via inlet 110.

The predominance of the secondary alcohol oxidation takes place in zone103, good mixing being provided by the high rate of liquid circulation.The exothermic heat of reaction is removed by vaporization of a portionof the reaction mixture, removal of the vapor mixture via outlet 111 andcondensation of components of the removed vapor. As described withrespect to FIG. 1, the mixture from the condensation is separated,liquid is recycled to zone 103 via inlets 112 the circulated reactionliquid and feed oxygen are passed to zone 103 via inlets 110 while theoxygen containing vapor after separation of a purge stream is returnedto zone 104 via inlets 107.

The liquid level in cylindrical zone 103 tends to be higher than thelevel in annular zone 104 since zone 103 is essentially operated as aboiling reactor and appropriate level control means (not shown) shouldbe provided to prevent overflow from zone 103 to zone 104.

It is important that a substantial oxygen partial pressure, e.g. 10 psior more, be maintained in the vapor stream exiting the reactor viaoutlet 11 in order that high reaction rate and selectivity be achieved.

The reaction mixture in annular zone 104 is maintained at a lowertemperature than the temperature of the mixture in zone 103, usually3°-5° C. lower. In this way, excessive vaporization from this zone isprevented. The reaction taking place in annular zone 104 is moreselective, due to the lower peroxide concentration.

The following example illustrates practice of the invention.

EXAMPLE

In accordance with FIGS. 1, 2a and 2b, a spherical reactor 34 ft. indiameter is provided having a cylindrical baffle, 20 ft. in diameter,mounted therein secured at the bottom to the inner reactor wall asindicated in FIG. 2. Baffle height is 26 ft. and 21 holes having adiameter of 2 inches are evenly spaced at the baffle bottom.

Referring to FIG. 1, feed isopropanol passes via lines 5 into reactionzone 4 and recycle vapor from separator 11 passes via lines 6 to zone 4.The temperature of the reaction mixture in zone 4 is maintained at 145°C. by virtue of the feed isopropanol being introduced at a temperatureof 120° C. The average residence time of the reaction mixture in zone 4is about 50 minutes and about 5% of the isopropanol feed to zone 4 isoxidized therein.

Reaction mixture flows from zone 4 to zone 3 via the holes at the bottomof baffle 2 as above described. Also fed to zone 3 is the liquidcondensate from separator 11 which passes to zone 3 via line 12. Astream of liquid reaction mixture is continuously removed from zone 3via lines 15, a net liquid product stream being separated via line 16with the remainder of the liquid reaction mixture circulating via pump17 and line 7 back to zone 3.

Oxygen feed to the reaction system is provided via line 18. Inaccordance with the invention, the oxygen is sparged into the highvelocity circulating liquid reaction mixture wherein it is rapidlydispersed without the formation of possibly hazardous vapor pockets, andpasses to zone 3 along with the circulating liquid in line 7.

Good mixing is achieved in zone 3 by virtue of the introduction thereinof the oxygen and recycle liquid. Temperature in zone 3 is 150° C. andthe average residence time therein is 50 minutes.

Vapors are continuously withdrawn from reactor 1 via line 8 at a rateeffective to maintain the desired temperatures in zone 3. The reactorpressure is 150 psia and the oxygen partial pressure in the vaporsexiting via line 8 is 9 psi.

Methane ballast vapor is introduced via line 19 into the reaction vaporin line 8, and the resulting mixture passes to condenser 9 wherein it iscooled to 120° C. and thence to liquid/vapor separator 11. Uncondensedvapors are separated via line 13 with a purge stream taken via line 14and the remaining vapor (after a recompression which is not shown) isrecycled via lines 6 to zone 4 of reaction 1. Liquid condensate fromseparator 11 recycles via line 12 to zone 3 of reactor 1.

The following table shows the compositions and flow rates of the variousprocess streams.

                  TABLE                                                           ______________________________________                                                 Stream                                                                        5      6      15        16   7                                       ______________________________________                                        Flow rate, lbs/hr                                                                        525      50     3700    563  3132                                  Temp., °F.                                                                        200      278    300     300  300                                   Pressure, psia.                                                                          165      170    160     160  170                                   Composition, wt %                                                             IPA         87      35.4   63      63   63                                    O.sub.2    --       22.5   --      --   --                                    Acetone     1       26.9   16.8    16.8 16.8                                  H.sub.2 O.sub.2                                                                          --       --     7.7     7.7  7.7                                   Methane    --       9      --      --   --                                    H.sub.2 O   12      5.6    11.7    11.7 11.7                                  ______________________________________                                                 Stream                                                                        18     8      19        14   12                                      ______________________________________                                        Flow rate, lbs/hr                                                                        100      221     1      7    180                                   Temp., °F.                                                                        100      300    100     250  250                                   Pressure, psia                                                                           175      150    150     150  150                                   Composition, wt %                                                             IPA        --       58     --      35.4 62.9                                  O.sub.2    100      4.1    --      22.5 0.2                                   Acetone    --       25.4   --      26.9 24.8                                  H.sub.2 O.sub.2                                                                          --       0.5    --      --   0.6                                   Methane    --       1.7    100     9    0.3                                   H.sub.2 O  --       9.5    --      5.6  10.4                                  ______________________________________                                    

Practice of the invention is especially advantageous in that increasedyields of hydrogen peroxide are achieved while operation is conductedsafely and efficiently. Yields of hydrogen peroxide are readilyobtainable which are 5% or more higher than those achieved byconventional procedures.

I claim:
 1. A reactor for the oxidation of a secondary alcohol tohydrogen peroxide comprised of a central cylindrical reaction zone andan annular reaction zone surrounding the central zone, the zones beingseparated by a cylindrical baffle, said baffle having holes adjacent thelower end to permit liquid flow from the annular zone to the cylindricalzone, a common vapor space above the central and annular reaction zones,means for introducing liquid recycle and reactant streams to the annularreaction zone, means for withdrawing liquid reaction mixture from thecentral reaction zone, and means for withdrawing vapor from the commonvapor space above the central and annular reaction zones.
 2. Theapparatus of claim 1 wherein the reactor is spherical.